Anti-aliasing is a technique that is used to make graphics and text easier to read and pleasing to the eye when viewed on a computer screen. Anti-aliasing is a way of getting around the low dots per inch (DPI) resolution of the computer screen (such as 72 DPI). Anti-aliased rendering affects pixels on the edges of a rendered figure. It calculates the percentage of pixel area covered by the figure and composes a color value as a mixture of figure color and background color. Anti-aliasing cannot improve the physical resolution of a display (e.g. an anti-aliased image on 72 DPI display will never be as crisp as on 1200 DPI), but anti-aliasing minimizes the difference between ideal and generated images and thus improves quality. In particular, anti-aliasing suppresses so-called “saw teeth” edges that used to appear on tilted figure edges without anti-aliasing.
One problem with anti-aliasing, however, is that it can produce irregular soft and sharp rendering of an image on the screen, depending on the location of edges. For example, an edge that falls exactly between screen pixels appears sharp, but an edge that falls in the middle of a screen pixel appears soft. This problem is implicitly created by device independent layout calculations. When device resolution is known, layout calculations can place edges exactly onto the boundaries between pixel rows and columns. Device independency assumes the resolution is not known so edges will either coincide with the pixel grid or take some fraction of a pixel row. For example, a vertical black line that is one pixel wide can appear as a column of black pixels while another similar line can appear as two columns of grey pixels, due to half pixel offset.
An additional problem that can occur when using anti-aliasing is seeping. Seeping occurs when two abutting objects have a common edge that is not aligned between a row or column of device pixels. The first rendered object fills the pixels on the edge with a mixture of background and foreground color. The second object will take these pixels as its background and in turn mix it with a second foreground color so that the initial background penetrates into the resulting color values. Seeping is especially unpleasant when objects have the same solid foreground color and are supposed to appear as a single shape, while anti-aliased rendering leaves undesired seams that remain visible even on high DPI devices.